The discovery of novel adeno-associated viruses (AAV) and their development as vectors has been an important accomplishment of this project in the first two cycles of this program project. While we are encouraged by the improved performance profiles of the novel AAV based vectors there is much to be learned before they can be responsibly evaluated in humans. This project will identify the AAV capsid that confers the most attractive performance profile as a vector following liver directed gene transfer. Key issues of vector immunology which may impact on safety and efficacy will be thoroughly characterized. Specific Aim #1 will evaluate the potential of two possible AAV clinical candidates for potential applications in humans. The first set of studies will measure gene transfer efficiency into human hepatocytes grown as xenografts in immune deficient mice. The AAV Clinical Candidate also will be evaluated in murine models of familial hypercholesterolemia (FH) and abetalipoproteinemia (ABL) for gene transfer efficiency, toxicity and vector immunogenicity. The second and third specific aims will evaluate an hypothesis that emerged from a phase I clinical trial with AAV serotype 2 in subjects with hemophilia B. These investigators proposed that the input capsid proteins of the vector activated cytotoxic T lymphoctyes (CTLs) that target and kill transduced hepatocytes leading to loss of transgene expression and self-limited hepatitis. This grant will evaluate in mice and monkeys the potential of the Clinical Candidate to active T cells and the role of memory T cells to the capsid in this activation step. Experiments will also be directed to the second key aspect of this hypothesis which is the cross presentation of input capsid by the hepatocytes to render them targets for CTLs. In an attempt to study this key step in the most relevant biological setting, experiments will be performed in xenograft mice that are reconstituted with human CTLs specific to an epitope on the capsid; the mice will be engineered to express a human HLA in hepatocytes. A fully humanized model will be developed in which animals are reconstituted with both human liver and human T cells to study cross presentation of capsid and T cell targeting.